Remove legacy object creation mode, and convert remaining uses of it
Register snapshots
auto merge of #5177 : catamorphism/rust/register-snapshots, r=catamorphism

Note that this throws FreeBSD under the bus. Hopefully we'll pull it back out again soon.
Remove code that was awaiting a snapshot

* Disallow structural records everywhere
* Remove all #[cfg(stage0)] stuff
* Remove the last deprecated modes in libcore
* Un-xfail a test
auto merge of #5147 : nikomatsakis/rust/remove-legacy-trait-table, r=nikomatsakis

r? @pcwalton
testsuite: Re-xfail record-trailing-comma

It was xfailed before the other commits in this pull request,
so no big deal. I'll look into it later.
auto merge of #5180 : catamorphism/rust/post-snapshot, r=catamorphism

* Disallow structural records everywhere
* Remove all #[cfg(stage0)] stuff
* Remove the last deprecated modes in libcore
* Un-xfail a test
Change the default level of unused_imports to warn
Remove unused imports throughout src/
Add a special FIXME for subduing warnings in rpath.rs
Adding missing imports for tests, and gate off others